Abnormal cryogenic strength enabled by magnetic-ordering-driven tetragonality in metastable steel

準安定鋼における磁気秩序駆動による正方晶性によって可能になる異常な極低温強度

諸岡 聡  ; 井川 直樹   ; 佐々木 未来; 生田目 望; 樹神 克明   

Morooka, Satoshi; Igawa, Naoki; Sasaki, Miki; Nabatame, Nozomi; Kodama, Katsuaki

Medium Mn steels have been actively investigated due to their excellent balance between material cost and mechanical properties. In particular, medium Mn steel with a nominal chemical composition of Fe-5.0Mn-0.1C (mass%) fabricated by intercritical annealing 923 K for 1.8 ks after cold-rolling, was the high-strength mechanical properties at low temperature. This strengthening mechanism evaluated by means of in-situ neutron diffraction under low temperature (High Resolution Powder Diffractometer (HRPD) at Japan Research Reactor-3(JRR-3)), electron back scatter diffraction (EBSD), low temperature differential scanning calorimetry (DSC) and low temperature magnetic susceptibility measurement. We found that as the sample temperature decreases, face- centered cubic (FCC) structure transferred face-centered tetragonal (FCT) structure. Namely, it suggests that austenite transformed martensite like Fe-Pd or Fe-Pt alloy. Therefore, the origin of the high-strength mechanical properties at low temperature was in the presence of FCT martensite.